Amp draw on battery

Posted By: wtucker

Amp draw on battery - 02/17/13 05:23 PM

I'm trying to determine if I have a unknown draw on my battery. I disconnected the negative cable and put an amp meter between the cable and negative battery post. I measured .04 amps on a 10 amp scale. Is that good or bad?

Posted By: Pacnorthcuda

Re: Amp draw on battery - 02/17/13 05:31 PM

That's 40ma (milliamperes) very little. Very normal.

Posted By: wtucker

Re: Amp draw on battery - 02/17/13 05:40 PM

Should there be measurable voltage between the negative cable and post?

Posted By: 68_CONV_300

Re: Amp draw on battery - 02/17/13 05:46 PM

if you are drawing a constant 40 milliamps, then you disconnect the negative cable from the post, and measure the voltage between the post and cable then yes you will show a voltage difference.

Posted By: wtucker

Re: Amp draw on battery - 02/17/13 05:49 PM

What should be the maximum voltage draw be between the negative cable and negative post?

Posted By: rarefish

Re: Amp draw on battery - 02/17/13 05:53 PM

If you put a volt meter in series between the post and cable you will not read anything even if there is current flow. Voltage is read across a load (parallel) and amperage is read in series.

Posted By: wtucker

Re: Amp draw on battery - 02/17/13 06:01 PM

I,m reading 12.55 volts between the negative cable and negative post. I assume this is not normal?

Posted By: rarefish

Re: Amp draw on battery - 02/17/13 06:10 PM

That is more like the voltage you would read across the battery terminals. Even if something is wrong there still should be no voltage reading when you put the meter in series with the cable and terminal.

Posted By: 68_CONV_300

Re: Amp draw on battery - 02/17/13 06:17 PM

Quote:

What should be the maximum voltage draw be between the negative cable and negative post?

Voltage is a measurement of potential work to be done , there is no draw associated with it.

think voltage as water pressure

think of current(amps) as the volume of water

think of the size of a pipe as the resistance in a circuit

if you have a steady water pressure (volts) and a 1/2" pipe(resistance) there will be a certain flow (amps) rate.

if you increase the pressure (voltage) but keep the pipe (resistance) the same size you will have more flow (amps)

if you keep the pressure(voltage) the same but increase the pipe size (resistance) you will also get more flow (amps)

I think you were asking how many volts should you read between the cable and the post when disconnected. and that question can only be answered by having a resistance reading from the positive cable to the negative cable.

Posted By: 68_CONV_300

Re: Amp draw on battery - 02/17/13 06:22 PM

Quote:

I,m reading 12.55 volts between the negative cable and negative post. I assume this is not normal?

this is perfectly normal ,the reason you are getting this reading is because there is a current draw with less resistance than the internal resistance of the meter. what you are creating by doing what you are doing is creating a circuit that is a called a voltage divider, and there is no way I'm going to explain it in a post here.

you need to be concerned with the current draw you are getting with the meter connected in series and in amp reading mode.

your reading of 40 milliamps is normal for a car with any type of electronics or an electric clock in it.

Posted By: 383man

Re: Amp draw on battery - 02/17/13 06:43 PM

40 milliamps is normal as was said. On the newer Mopars I used to check all the time the average was about 20 milliamps once all controllers powered down. Up to 50 is normal and will not cause a problem unless the car sitts for weeks at at a time. One thing to remember is some controllers can take up to 20 minutes to power down. I have tested a few that took just over 10 minutes to power down. Ron

Posted By: Supercuda

Re: Amp draw on battery - 02/17/13 07:06 PM

Quote:

That is more like the voltage you would read across the battery terminals. Even if something is wrong there still should be no voltage reading when you put the meter in series with the cable and terminal.

Wrong, he's reading a normal reading for that. Though you really shouldn't measure voltage that way it is normal.

Posted By: rarefish

Re: Amp draw on battery - 02/17/13 07:23 PM

Quote:

Quote:

That is more like the voltage you would read across the battery terminals. Even if something is wrong there still should be no voltage reading when you put the meter in series with the cable and terminal.

Wrong, he's reading a normal reading for that. Though you really shouldn't measure voltage that way it is normal.

Now you got me wondering... so I just went out into a very cold garage and popped the hood on my Dakota. I pulled the neg terminal off the battery and put my meter in series and read 12.3 vdc

Well now I'm surprised. Anyway I also checked curent flow and found that I had .04 ma

I stand corrected on the voltage statement that I made earlier.

Posted By: Pacnorthcuda

Re: Amp draw on battery - 02/17/13 07:36 PM

Quote:

Quote:

Quote:

That is more like the voltage you would read across the battery terminals. Even if something is wrong there still should be no voltage reading when you put the meter in series with the cable and terminal.

Wrong, he's reading a normal reading for that. Though you really shouldn't measure voltage that way it is normal.

Now you got me wondering... so I just went out into a very cold garage and popped the hood on my Dakota. I pulled the neg terminal off the battery and put my meter in series and read 12.3 vdc
Well now I'm surprised. Anyway I also checked curent flow and found that I had .04 ma

I stand corrected on the voltage statement that I made earlier.

Ya might want to put a charger on that battery.

Posted By: rarefish

Re: Amp draw on battery - 02/17/13 07:59 PM

I did check the battery across the terminals and it read 12.45 vdc. I was thinking that reading might be a little low.

Posted By: 383man

Re: Amp draw on battery - 02/17/13 11:17 PM

If you unhook the neg or pos cable and put a voltmeter in series with it you will not get an accurate voltage reading. As you said just 40 milliamps had the voltmeter reading 12 volts. Thats because a voltmeter needs to hooked in paralell in a circuit to read right. When you remove a battery cable and hook a voltmeter to the battery post and the cable you are hooking the voltmeter in series which wont read the correct voltage. Many guys used to use test lights in series with a cable but thats not an accurate way either. Thats why you use the ammeter to check the draw as thats the only way to get accurate reading. Ron

Posted By: wtucker

Re: Amp draw on battery - 02/18/13 01:39 PM

So what would be drawing .04amps when there isn't anything hooked up. No clock, radio, nothing digital, no dome light, or is that just a normal slow discharge of a typical battery.